1. Field of the Disclosure
The present invention relates in general to an electrophotographic imaging apparatus and in particular to an electrophotographic apparatus which controls throughput based on media width using temperature feedback.
2. Description of the Related Art
In an electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to a media sheet intended to receive the final image. The toner image is fixed to the media sheet by the application of heat and pressure in a fuser assembly. The fuser assembly may include a heated roll and a backup roll forming a fuser nip through which the media sheet passes. Alternatively, the fuser assembly may include a fuser belt, a heater disposed within the belt around which the belt rotates, and an opposing backup member, such as a backup roll.
To be able to fuse the widest media that the laser printer is designed to print, the length of the heating region is typically about 2 mm to about 3 mm longer than the width of the widest media supported by the printer. When a to-be-printed media sheet has a width narrower than the width of the widest media supported by the printer, an overheating problem may occur. Along the portion of the fuser which does not contact the narrow media as the narrow media passes through the fuser, the fluoropolymer coated belt and backup roll of the fuser become very hot and can be damaged due to the high temperature. FIG. 7 below explains the cause of this problem.
FIG. 7 illustrates wide (e.g. Letter) and narrow (e.g., A5) media passing through the fuser nip N of a fuser for a reference edged fed imaging device. Wide media is illustrated in the top part of FIG. 7 and narrow media the bottom part thereof. As is apparent from FIG. 7, a portion of the fuser nip N, Region A, is not contacted by the narrow media. In this case, heat generated by the ceramic heater is not removed from Region A by the narrow media thereby causing an overheating problem. Without heat being removed from Region A, heat generated in Region A continues to heat the coated belt and the backup roll of the fuser. Further, laser printers are designed to have a very small first-copy time such that the thermal mass of the heater and of the coated belt is very small. This causes the amount of heat to build up rapidly in the heater and coated belt in Region A. Furthermore, to achieve a small first-copy time and sufficiently fix the toner to the media sheet, the backup roll surface desirably becomes very hot without conducting heat to the steel or aluminum shaft of the backup roll. This is achieved because the layer surrounding the backup roll shaft is rubber which is a thermal insulator. However, this also means the heat conducted away from the coated belt and heater in Region A by the backup roll is very small. The only other possible mechanism to significantly remove heat from Region A is air convection. Unfortunately, the amount of heat removed by convection is very small for two reasons: 1) in order to meet the very small first-copy time, the heat lost to the air is minimized by enclosing the coated belt and backup roll in plastic covers or a housing to keep the air still; and 2) such plastic covers are designed to act as a heat insulating surface, not a heat conducting device.
Since excessive thermal energy accumulated at the portion of the fuser not contacting the media (hereinafter “non-media portion”) during narrow media printing can cause damage to the fusing belt and backup roll, it is desirable to control the amount of thermal energy accumulated at the non-media portion to be below a certain level so that the fuser will not be damaged. To control the thermal energy accumulated at the non-media portion of the fuser, prior attempts both used one or multiple narrow media, mechanical flag sensors to detect media width and user-provided information to determine media length and weight. However, mechanical flag sensors are limited both in precision and being able to detect a number of different media widths, and user-provided information is oftentimes faulty. As a result, prior attempts either made media throughput decisions that were too conservative, thereby leading to reduced performance levels, or caused fuser overheating to occur.
Based on the foregoing, there is a need for an improved system for controlling fusing operations on narrow media sheets in an image forming apparatus.